The structure and functions of the receptor of sensory systems. The main functions of cell receptors

The human nervous system carries out complex analytical and synthetic processes that ensure the rapid adaptation of organs and systems to changes in the external and internal environment. The perception of stimuli from the outside world is due to a structure that includes the processes of afferent neurons containing glial oligodendrocyte cells or lemocytes. They transform external or internal stimuli into bioelectric phenomena, called excitation or nerve impulses. Such structures are called receptors. In this article, we will study the structure and functions of the receptors of various human sensory systems.

Types of nerve endings

In anatomy there are several systems for their classification. The most common divides the receptors into simple (consisting of the processes of one neuron) and complex (a group of neurocytes and helper cells in a highly specialized sense organ). Proceeding from the structure of the sensory processes. They are divided into primary and secondary endings of the centripetal neurocyte. These include various receptors of the skin: nociceptors, mechanoreceptors, baroreceptors, thermoreceptors, as well as nerve processes innervating internal organs. Secondary are derivatives of the epithelium, creating an action potential in response to stimulation (receptors for taste, hearing, balance). The sticks and cones of the photosensitive membrane of the eye - the retina - occupy an intermediate position between the primary and secondary sensory nerve endings.

Another classification system is based on such a difference as the type of stimulus. If the stimulation comes from the external environment, then it is perceived exteroceptors (eg sounds, smells). A stimulation by factors of the internal environment is analyzed by interoreceptors: visceral, proprioceptors, hair cells of the vestibular apparatus. Thus, the functions of receptors of sensory systems are due to their structure and location in the sense organs.

The concept of analyzers

In order to differentiate and differentiate the conditions of the external environment and adapt to it, a person has special anatomical and physiological structures called analyzers, or sensory systems. Russian scientist IP Pavlov proposed the following scheme of their structure. The first department was called peripheral (receptor). The second - conductor, and the third - the central, or cortical.

For example, the visual sensory system includes sensitive cells of the retina - rods and cones, two optic nerve, and a cortical area located in its occipital part.

Some analyzers, such as those already mentioned visual and auditory, include the pre-receptor level - certain anatomical structures that improve the perception of adequate stimuli. For the auditory it is the outer and middle ear, for the visual system - the refractory part of the eye, which includes the sclera, watery moisture of the anterior chamber of the eye, the lens, the vitreous. We will focus on the peripheral part of the analyzer and answer the question of what is the function of the receptors involved in it.

How cells perceive stimuli

In their membranes (or in the cytosol) are special molecules consisting of proteins, as well as complex complexes - glycoproteins. Under the influence of environmental factors, these substances change their spatial configuration, which serves as a signal for the cell itself and forces it to react adequately.

Some chemicals, called ligands, can affect the sensory processes of the cell, resulting in the formation of transmembrane ion currents. Plasmalemma proteins possessing receptive properties, together with carbohydrate molecules (i.e., receptors) perform the functions of annten - perceive and differentiate ligands.

Ionotropic channels

Another type of cellular receptors is ionotropic channels located in the membrane that can open or block under the influence of signal chemical substances, for example, H-cholinergic receptor, receptors of vasopressin and insulin.

Intracellular sensory structures include transcription factors that bind to the ligand and then penetrate the nucleus. They form compounds with DNA that enhance or inhibit the transcription of one or more genes. Thus, the main functions of the cell receptors are the perception of signals from the external environment and the regulation of the reactions of plastic metabolism.

Sticks and cones: structure and functions

These retinal receptors respond to light stimuli - photons, which cause the excitation process in the nerve endings. They contain special pigments: iodopsin (cones) and rhodopsin (rods). The rods are irritated by the twilight light and can not distinguish colors. The cones are responsible for color vision and are divided into three types, each containing a separate photopigment. Thus, the function of the eye's receptor depends on which photosensitive proteins it contains. Sticks cause visual perception in low light, and cones are responsible for visual acuity and color perception.

Skin is the organ of the senses

Nerve endings of neurons entering the dermis differ in their structure and react to various stimuli of the external environment: temperature, pressure, surface shape. Functions of skin receptors - to perceive and transform stimuli into electrical impulses (the process of excitation). Pressure receptors include the Meissner corpuscles located in the middle layer of skin - the dermis, capable of a subtle discrimination of stimuli (have a low threshold of sensitivity).

The barocereceptors include Pacini's body. They are located in the subcutaneous fat. Functions of the receptor - nociceptor pain - is protection from pathogenic irritants. In addition to the skin, such nerve endings are located in all internal organs and have the form of branching afferent processes. Thermoreceptors can be found both in the skin and in internal organs - blood vessels, parts of the central nervous system. They are classified into thermal and cold.

The activity of these sensory endings can increase and depends on the direction in which and at what speed the temperature of the skin surface changes. Consequently, the functions of the skin receptors are diverse and depend on their structure.

The mechanism of perception of auditory stimuli

Exteroreceptors are hair cells that have a high sensitivity to adequate stimuli - sound waves. They are called monomodal and are secondarily sensitive. Located in the organ of the cortex of the inner ear, entering the snail.

By its arrangement of corti the organ is like a harp. The auditory receptors are immersed in perilymph and have at their ends a group of microvilli. Fluctuations of the liquid cause irritation of the hair cells, turning into bioelectric phenomena - nerve impulses, ie, the function of the receptor of hearing - is the perception of signals that look like sound waves, and their transformation into the process of excitation.

Taste contact receptors

Each of us has a preference in food and drink. Taste range of food we perceive with the help of taste organ - language. It contains four types of nerve endings, localized as follows: on the tip of the tongue - taste buds that distinguish sweet, on its root - bitter, and salty and sour distinguish receptors of the side walls. Irritants for all types of receptor endings are molecules of chemical substances, perceived by microvilli of taste bulbs, performing the functions of antennas.

Functions of the receptor of taste - to decode a chemical stimulus and transfer it into an electrical impulse, arriving along the nerves in the taste zone of the cerebral cortex. It should be noted that the papillae work in conjunction with the nerve endings of the olfactory analyzer located in the mucous membrane of the nasal cavity. The joint action of the two sensory systems enhances and enriches the taste sensations of a person.

The Mystery of the Smell

Just like the taste, the olfactory analyzer reacts with its nerve endings to molecules of various chemicals. The mechanism by which odoriferous compounds irritates olfactory bulbs, until the end has not been studied. Scientists suggest that the signal molecules of the odor interact with various sensory neurons of the nasal mucosa. Other researchers associate the irritation of olfactory receptors with the fact that signaling molecules have common functional groups (for example, aldehyde or phenolic) with substances that enter the sensory neuron.

The functions of the olfactory receptor are the perception of irritation, its differentiation and translation into the excitation process. The total number of olfactory bulbs in the mucosa of the nasal cavity reaches 60 million, each of which is provided with a large number of cilia, due to which the total area of contact of the receptor field with molecules of chemical substances - odors increases.

Nerve endings of the vestibular apparatus

In the inner ear is the organ responsible for coordination and coordination of motor acts, maintaining the body in a state of equilibrium, and also participating in orientation reflexes. It has the form of semicircular canals, called the labyrinth and is anatomically linked to the Corti's organ. In the three bony canals there are nerve endings immersed in the endolymph. With the tilts of the head and torso, it oscillates, which causes irritation at the ends of the nerve endings.

The vestibular receptors, the hair cells themselves, come into contact with the membrane. It consists of small crystals of calcium carbonate - otoliths. Together with the endolymph they also begin to move, which serves as an irritant for the nerve processes. The main functions of the receptor of semicircular canals depend on its location: in pouches, it reacts to gravity and controls the equilibrium of the head and body at rest. Sensory endings, located in ampoules of the body of equilibrium, control the change in the movements of parts of the body (dynamic gravity).

The role of receptors in the formation of reflex arcs

The whole doctrine of reflexes, beginning with the research of R. Descartes and the fundamental discoveries of IP Pavlov and IM Sechenov, is based on the idea of nervous activity as an adequate response of the organism to the effects of stimuli of the external and internal environment, carried out with the participation of the central nervous System - the brain and spinal cord. Whatever the answer, simple, for example, a knee-jerk reflex, or so super complicated as speech, memory or thinking, its first link is the reception - perception and discrimination of stimuli by their strength, amplitude, intensity.

Such differentiation is realized by sensory systems, which IP Pavlov called "tentacles of the brain". In each analyzer, the receptor functions as antennas that capture and probe environmental stimuli: light or sound waves, molecules of chemical substances, physical factors. The physiologically normal activity of all sensory systems without exception depends on the work of the first department, called peripheral, or receptor. All reflex arcs (reflexes) originate from it without exception.

Mediators

These are biologically active substances that transfer excitation from one neuron to another in special structures - synapses. They are secreted by the axon of the first neurocyte and, acting as an irritant, cause nerve impulses in the receptor endings of the next nerve cell. Therefore, the structure and functions of the mediators and receptors are closely interrelated. Moreover, some neurocytes are capable of secreting two or more transmitters, for example, glutamic and aspartic acids, epinephrine and GABA.